Whenever you can afford it.  The bitcoin exchange rate is highly volatile, and this is still an experimental system.  Do not store value in bitcoin that you would be devastated to lose.


Until you need to spend it, or until you decide that you no longer have faith in the future of bitcoin, whichever comes first.


Why would you want to waste time?  Time is the most valuable thing you will ever have in your life. As for earning bitcoins, bitcoin is money so you get it the exact same way as you get any other form of money:

• Exchange your local currency for bitcoins
• Provide some product or service, and accept bitcoins in exchange
• Receive bitcoin as a gift out of the charity in someone's heart
• Scam, blackmail, extort, steal, hack, or any other illegal activity to acquire bitcoins

If you want to mine as a hobby, and are willing to lose money to do so, then you can pick up just about any ASIC at a reasonable price and start mining.  There is a mining sub-forum here at bitcointalk where you can get lots of information about how to connect and configure the equipment and keep it running optimally.

If you want to mine profitably, then you need inexpensive access to the most efficient mining equipment and you need electricity costs that are competitive with the largest mining operations in the world. There are many people who *think* they are mining profitably, but most of them are just bad at math.  They fail to take into consideration some of their costs, or some of their lost opportunities.